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A leaf (plural leaves ) is the principal lateral appendage of the vascular plant stem, usually borne above ground and specialized for photosynthesis. The leaves and stem together form the shoot. Leaves are collectively referred to as foliage, as in "autumn foliage". In most leaves, the primary photosynthetic tissue, the palisade mesophyll, is located on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of Eucalyptus , palisade mesophyll is present on both sides and the leaves are said to be isobilateral. Most leaves are flattened and have distinct upper ( adaxial ) and lower ( abaxial ) surfaces that differ in color, hairiness, the number of stomata (pores that intake and output gases), the amount and structure of epicuticular wax and other features. Leaves are mostly green in color due to the presence of a compound called chlorophyll that is essential for photosynthesis as it absorbs light energy from the sun. A leaf with

Leaves are the most important organs of most vascular plants. Green plants are autotrophic, meaning that they do not obtain food from other living things but instead create their own food by photosynthesis. They capture the energy in sunlight and use it to make simple sugars, such as glucose and sucrose, from carbon dioxide and water. The sugars are then stored as starch, further processed by chemical synthesis into more complex organic molecules such as proteins or cellulose, the basic structural material in plant cell walls, or metabolized by cellular respiration to provide chemical energy to run cellular processes. The leaves draw water from the ground in the transpiration stream through a vascular conducting system known as xylem and obtain carbon dioxide from the atmosphere by diffusion through openings called stomata in the outer covering layer of the leaf (epidermis), while leaves are orientated to maximize their exposure to sunlight. Once sugar has been synthesized, it needs to

A structurally complete leaf of an angiosperm consists of a petiole (leaf stalk), a lamina (leaf blade), stipules (small structures located to either side of the base of the petiole) and a sheath. Not every species produces leaves with all of these structural components. The proximal stalk or petiole is called a stipe in ferns. The lamina is the expanded, flat component of the leaf which contains the chloroplasts. The sheath is a structure, typically at the base that fully or partially clasps the stem above the node, where the latter is attached. Leaf sheathes typically occur in grasses and Apiaceae (umbellifers). Between the sheath and the lamina, there may be a pseudopetiole, a petiole like structure. Pseudopetioles occur in some monocotyledons including bananas, palms and bamboos. Stipules may be conspicuous (e.g. beans and roses), soon falling or otherwise not obvious as in Moraceae or absent altogether as in the Magnoliaceae. A petiole may be absent (apetiolate), or the blade may

Medium-scale features edit Leaves are normally extensively vascularized and typically have networks of vascular bundles containing xylem, which supplies water for photosynthesis, and phloem, which transports the sugars produced by photosynthesis. Many leaves are covered in trichomes (small hairs) which have diverse structures and functions. Small-scale features edit The major tissue systems present are The epidermis , which covers the upper and lower surfaces The mesophyll tissue inside the leaf, which is rich in chloroplasts (also called chlorenchyma ) The arrangement of veins (the vascular tissue) These three tissue systems typically form a regular organization at the cellular scale. Specialized cells that differ markedly from surrounding cells, and which often synthesize specialized products such as crystals, are termed idioblasts . Major leaf tissues edit Epidermis edit The epidermis is the outer layer of cells covering the leaf. It is covered with a waxy cuticle which is impe

According to Agnes Arber's partial-shoot theory of the leaf, leaves are partial shoots, being derived from leaf primordia of the shoot apex. Early in development they are dorsiventrally flattened with both dorsal and ventral surfaces. Compound leaves are closer to shoots than simple leaves. Developmental studies have shown that compound leaves, like shoots, may branch in three dimensions. On the basis of molecular genetics, Eckardt and Baum (2010) concluded that "it is now generally accepted that compound leaves express both leaf and shoot properties."

Biomechanics edit Plants respond and adapt to environmental factors, such as light and mechanical stress from wind. Leaves need to support their own mass and align themselves in such a way as to optimize their exposure to the sun, generally more or less horizontally. However, horizontal alignment maximizes exposure to bending forces and failure from stresses such as wind, snow, hail, falling debris, animals, and abrasion from surrounding foliage and plant structures. Overall leaves are relatively flimsy with regard to other plant structures such as stems, branches and roots. Both leaf blade and petiole structure influence the leaf's response to forces such as wind, allowing a degree of repositioning to minimize drag and damage, as opposed to resistance. Leaf movement like this may also increase turbulence of the air close to the surface of the leaf, which thins the boundary layer of air immediately adjacent to the surface, increasing the capacity for gas and heat exchange, as well

In the course of evolution, leaves have adapted to different environments in the following ways: citation needed Waxy micro- and nanostructures on the surface reduce wetting by rain and adhesion of contamination ( See Lotus effect ). Divided and compound leaves reduce wind resistance and promote cooling. Hairs on the leaf surface trap humidity in dry climates and create a boundary layer reducing water loss. Waxy plant cuticles reduce water loss. Large surface area provides a large area for capture of sunlight. In harmful levels of sunlight, specialized leaves, opaque or partly buried, admit light through a translucent leaf window for photosynthesis at inner leaf surfaces (e.g. Fenestraria ). Kranz leaf anatomy in plants who perform C4 carbon fixation Succulent leaves store water and organic acids for use in CAM photosynthesis. Aromatic oils, poisons or pheromones produced by leaf borne glands deter herbivores (e.g. eucalypts). Inclusions of crystalline minerals deter herbivor

Shape edit Edge (margin) edit Image Term Latin Description Entire Forma integra Even; with a smooth margin; without toothing Ciliate Ciliata Fringed with hairs Crenate Crenata Wavy-toothed; dentate with rounded teeth Dentate Dentata Toothed May be coarsely dentate , having large teeth or glandular dentate , having teeth which bear glands Denticulate Denticulata Finely toothed Doubly serrate Duplicato-dentata Each tooth bearing smaller teeth Serrate Serrata Saw-toothed; with asymmetrical teeth pointing forward Serrulate Serrulata Finely serrate Sinuate Sinuosa With deep, wave-like indentations; coarsely crenate Lobate Lobata Indented, with the indentations not reaching the center Undulate Undulata With a wavy edge, shallower than sinuate Spiny or pungent Spiculata With stiff, sharp points such as thistles Apex (tip) edit Image Term Latin Description Acuminate _ Long-pointed, prolonged into a narrow, tapering p